This invention relates to a diamond-coated sintered body excellent in adhesion strength comprising a film of diamond or diamond-like carbon deposited on the surface of a substrate of a ceramics sintered body composed mainly of tungsten carbide and a process for preparing the same. More specifically, it relates to a diamond-coated sintered body excellent in adhesion strength which is suitable as the material for parts to be used in electrical industries, electronic industries, precise instrument industries, office instrument industries, etc. primarily for, for example, material for cutting tool, material for abrasion resisting tool or materials for decoration, and a process for preparing the same.
Investigations have been made in the prior art about practical application of a diamond coated sintered body having film of diamond and/or diamond-like carbon deposited on the surface of a substrate comprising a sintered body of a metal, an alloy or ceramics. Such diamond-coated sintered body, because diamond is inferior in chemical bonding with other substances, involves the problem as the greatest task how the film of diamond and/or diamond-like carbon can be coated on the surface of the substrate under the state enhanced in adhesion. Particularly, in the case of materials for cutting tool, milling tools, drills and end mills, since they are used under the severest conditions, adhesion strength between the film and the substrate will become further important problem.
There have been a large number of proposals about use of diamond-coated sintered bodies which are enhanced in adhesion strength between the film of diamond and the substrate and can be used as the material for cutting tools, and they have comprised the representative examples which have made investigations from the aspect of the material of the substrate, wherein Japanese Provisional Patent Publications No. 126972/1983, No. 57802/1987, No. 166904/1987 and No. 99102/1988, and as a representative example which has made investigation from the aspect of the work strain on the surface of the substrate, there is Japanese Patent Publication No. 4586/1989.
Japanese Provisional Patent Publication No. 126972/1983 discloses a cemented carbide tool comprising at least one selected from carbides, nitrides, borides and oxides of the groups 4a, 5a and 6a elements and their compounds, mixtures thereof, or Al.sub.2 O.sub.3, AlN, B.sub.4 C, SiC, Si.sub.3 N.sub.4, SiO.sub.2, as inner layer adjacent to the surface of a cemented carbide, and comprising diamond as outer layer. In this time, cemented carbide as the substrate comprises at least one carbide and nitride. The invention according to this Japanese Provisional Patent Publication No. 126972/1983 attempts to solve the problem that diamond is transformed to graphite when a diamond is coated directly on a cemented carbide containing Co or Ni, by interposing an intermediate layer containing no metal such as Fe, Co and Ni, for example, tungsten carbide between the cemented carbide and the diamond film. However, although there is no great problem with respect to the adhesion strength between the intermediate layer formed by the chemical vapor deposition method (CVD method) or the physical vapor deposition method (PVD method) and the substrate, but adhesion strength between the diamond film and the intermediate film is inferior to make practical application impossible. Also, the invention disclosed in the same Publication involves the problem of cumbersomeness of the process that the diamond film is required to be formed in an entirely separate reaction vessel after coating of the intermediate layer by the CVD method or the PVD method in order to form a diamond film with good quality. Also, as concerned with such cumbersomeness of the process, it has a problem that impurities are readily attached at the interface between the intermediate layer and the diamond film, whereby the adhesion strength of the diamond film, is lowered.
Japanese Provisional Patent Publication No. 57802/1987 discloses a hard carbon coated part, comprising a compound thin film of W and C composed mainly of W.sub.2 C with a thickness of 0.1 .mu.m or more interposed between the hard carbon thin film and the substrate. The invention according to the same Publication has accomplished the improvement of adhesion strength as the result of formation of a diffusion layer generating WC at the interface between hard carbon and W.sub.2 C when the intermediate layer as described above is coated on the surface of a substrate comprising a cemented carbide or ceramics, and subsequently hard carbon is coated on the surface of an intermediate layer, but it has the problem of cumbersomeness of the process that the hard carbon film is required to be formed in an entirely separate reaction vessel after formation of the intermediate layer by the CVD method or the PVD method, and there is also a problem that adhesion strength between the intermediate layer and the hard carbon thin film is inferior because impurities are readily attached on the intermediate surface.
Japanese Provisional Patent Publication No. 166904/1987 discloses a hard carbon film coated cutting tool for working of ceramics materials, comprising a substrate of a ceramics sintered body composed mainly of silicon nitride, silicon carbide, zirconium oxide, aluminum oxide, and a thin film of hard carbon coated to 0.5 to 50 .mu.m under low pressure on the surface thereof. The invention in the same Publication enables the cutting of a fine ceramics which is a difficultly cuttable material by use of a ceramics with high deformation resistance as the substrate and coating a thin film of hard carbon on the surface thereof, but, for example, there is a problem that adhesion strength between the substrate and the thin film deteriorates as the content of diamond in the hard carbon is increased, while abrasion resistance is lowered, if on the contrary, the content of diamond is decreased.
Japanese Provisional Patent Publication No. 53269/1988 discloses a diamond-coated chip in cutting tool comprising a substrate of cemented carbide containing 1 to 4% by weight of Co and the balance of tungsten carbide in this substrate having a structure with a grain size of 2 to 10 .mu.m. The invention in Japanese Provisional Patent Publication No. 53269/1988, as different from those having the intermediate layer of WC or W.sub.2 C between the diamond film or hard carbon film and the substrate in Japanese Provisional Patent Publications No. 126972/1983 and No. 57802/1987 as described above, comprises a diamond film deposited on the surface of the etching layer formed by etching Co existing on the surface of a cemented carbide by the acid treatment, and is free from the problem of cumbersomeness in the process and the deterioration of adhesion strength between the etching layer and the substrate. However, there are involved the problems that if the thickness of the etching layer becomes thick, the portion of Co removed by etching may sometimes remain as a void, whereby adhesion strength between the diamond film and the etching layer may be extremely lowered. To the contrary, if the thickness of the etching layer becomes thin, the generation of graphite during diamond deposition cannot be prevented enough.
Japanese Provisional Patent Publication No. 99102/1988 discloses a diamond coated tungsten tool, comprising tungsten as the substrate and diamond film coated to 1 to 100 .mu.m on said substrate. This invention in the same Publication has solved the problem of adhesion strength by use of tungsten as the substrate, in view of the diamond-coated tool of the prior art comprising a diamond thin film coated on the substrate surface of a cemented carbide or ceramics having the problem in adhesion strength between the thin film and the substrate. However, since tungsten itself is soft and hence susceptible to plastic deformation, whereby there is the problem that its life is short when used as the cutting tool.
Japanese Patent Publication No. 4586/1989 discloses a method in which the working strain is generated on the surface of cemented carbide by grinding with a grinding wheel for deposition of diamond, the respective work strain before and after grinding are determined as a half value widths of the X-ray diffraction line at the WC (100) crystal face, and subsequently, diamond film is coated on the surface of the substrate with its certain ratio. The invention in the same Publication resides in remarkable increase of the crystal nucleus of diamond by depositing a diamond on the substrate surface having a certain amount of work strain, whereby the film with fine structure is formed and also the adhesion strength between the film and the substrate is excellent. However, since VIII group metal in the periodic table is contained in the cemented carbide of the substrate, there are problems such that graphite is readily formed on the surface of the substrate at the initial stage during diamond deposition.